Such an articular prosthesis is to be implanted in a cavity made to this end in the bone concerned, the hip bone when a coxo-femoral joint is in question.
It must therefore comply with a double imperative: on the one hand, its outer surface must be provided with means adapted to allow a firm implantation thereof in the corresponding osseous cavity and, on the other hand, it must present, inside, at cotyloid cavity level, good surface and mechanical resistance qualities suitable for a cooperation, without noteworthy friction and without too rapid wear, with the spheroid head with which it is associated.
The implantation of this articular prosthesis may be effected, at the present time, by sealing or embedding or by screwing.
In the case of an implantation by sealing or embedding, it is preferable, for a better fixing in the osseous cavity concerned, if the articular prothesis in question has a rough outer surface, for example a porous macroscopic roughness outwardly open by pores, promoting growth and consolidation of the bone, as is the case of so-called "madreporic" cotyloid prostheses.
In the case of an implantation by screwing, it is obviously necessary that the articular prosthesis in question has at least one outer helical thread, as is the case in particular in French patent application filed on Feb. 27, 1974 under No. 74 42974 and published under No. 2,295,729, this helical thread furthermore being, in certain cases, self-tapping.
In view of the surface and mechanical resistance qualities that an articular prosthesis must have at cotyloid cavity level, this articular prosthesis is presently most often made of synthetic material, for example polyethylene, and even irradiated polyethylene, but it has been proposed to make them of ceramics, for example fritted alumina ceramics, as is the case in particular in French patent filed July 10, 1970 under No. 70 25848 and published under No. 2,096,895, to use the inherent qualities, known for a long time, of such a material.
In fact, ceramic materials, and this is also the case of monocrystalline materials, present, with respect to conventional metallic materials, incomparable advantages of considerable hardness, reduced coefficient of friction, inalterability, biological compatibility with the osseous tissues, and compressive strength.
On the other hand, due to their relatively low resilience and tensile strength, they are fragile to shocks.
In addition, they are not easy to machine and, in particular, it is difficult to make surface roughness or screw threads thereon.
The articular prostheses of the type in question are at the present time most often made in one piece.
For the above reasons, it is in practice difficult to give them a rough surface or threads when they are made of ceramics.
It has been proposed to form such an articular prosthesis with the aid of two separate parts, namely an inner cupule made of synthetic material, and an outer cupule made of metal, as is the case in particular in French patent filed on Jan. 7, 1975 under No. 75 00356 and published under No. 2,297,030, for an articular prosthesis to be implanted by sealing or embedding, and a similar arrangement has been adopted for articular prostheses to be screwed, the screw thread thus being formed on a less fragile metal part, namely an outer ring, and the cotyloid cavity on a part made of a material having better qualities of friction than metal, namely an inner cupule made of synthetic material.
However, in both cases, the inner cupule is made of a synthetic material, i.e. a material having surface and mechanical resistance qualities which are inferior to those of a ceramic material or a monocrystalline material and furthermore unlike the latter, capable of an untimely creep, due to the appreciable elasticity that it presents.
In addition, in both cases, the positioning of the articular prostheses in question does not include any disposition for promoting a rapid regeneration of the osseous tissue in which they are implanted.